Mass spectrometry



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HTTPNEYI H. W. WASHBURN MASS SPECTROMETRY 2 SHEETS-SHEET 2 OriginalFiled April 28, 1945 SQ wma Ei 3u 1N VEN TOR. Miam n/ Wmv/UEM ReissuedFeb. 26, 1952 UNITED STATES PATENT OFFICE MASS SPECTROMETRY Harold W.Washburn, to Consolidated Pasadena, Calif., assignor EngineeringCorporation,

Pasadena, Calif., a corporation of California Original No. 2,475,653,dated July 12, 1949, Serial No. 590,917, April issue October 26,

1951, Serial No.

Application for re- 14 Claims. (Cl. Z50-41.9)

Matter enclosed in heavy This invention is concerned with massspectrometry. It provides improvements in methods of and apparatus forforming an ion beam from gaseous molecules and propelling the beam intoan analyzer chamber. to the end that the stability of spectra and theaccuracy and rapidity of analysis is improved.

A mass spectrometer is essentially an apparatus for producing ions andsorting them according to the ratio of their mass to their charge, i. e.according to their specific mass. A sample to be analyzed, for example agas mixture, is ionized (for example, by electron bombardment) and theresulting ions are propelled by an electrical potential into ananalyzer. The ions enter the analyzer as an unsorted beam, and in theanalyzer are sorted under the influence of an electro-magnetic field inaccordance with their specific mass into a series of divergenthomogeneous ion beams. The sorted ions are collected and discharged, thequantity of each kind of ions being measured by the amount of currentthat they discharge upon collection.

In one type of mass spectrometer, ionization is accomplished bybombarding molecules of the sample in an ionization chamber with anelectron beam. The resulting ions are forced through a plurality ofcollimating electrodes having oriented slits or apertures therein, andemerge into an analyzer tube as an unsorted beam. It has been proposedheretofore to form the ions within the chamber and to propel them intothe collimating aperture in the first collimating electrode by means ofa potential maintained across the zone of ion formation by means of apusher electrode. The beam thus formed emerges from the slit of thefirst collimating electrode and is propelled to the slit in the secondcollimating electrode by a potential maintained between the two and ithasbeen proposed heretofore to aid collimation by one or more auxiliaryor focussing electrodes maintained at a potential intermediate those ofthe collimating electrodes and disposed between the collimatingelectrodes but to one or both sides of the ion beam. In this fashion atransverse component of propelling potential may be produced andemployed to vary the width of beam and to aid in collimation bypreventing ion paths in the beam from crossing over each other be-4tween the collimating electrodes.

As the result of my investigations, I have discovered thatimprovedresults are 'obtained with a mass spectrometer if the pusherelectrode is. in edect, eliminated so that there is no substantialpotential behind the zone of electron brackets I: appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

2 bombardment in which the ions are formed, but instead the potential isapplied to the ions im` mediately after they diffuse from the region oftheir formation. Thus my invention contemplates in a mass spectrometer,the combination l which comprises an ionization chamber, an inletelectrode disposed within the chamber and having an inner end providedwith a. passage for admission of molecules into the chamber with a slottransverse to the passage, a second electrode spaced from the inletelectrode, means for producing an electrical propelling potentialbetween the electrodes in the ionization chamber, and means forpropelling an electron beam through the slot transverse to the passage.In such an arrangement of apparatus the ions are formed by electronbombardment in a region outside the field of major ion propellingpotential and enter this field primarily as a, result of diffusion.Moreover, since ionization occurs in the restricted space of the slotrather than in the larger space within the ionization chamber, theelectron bombardment and consequent ionization takes place at a pointwhere the sample is relatively highly concentrated and a -higherpercentage of the entering molecules are given an opportunity to ionize.This increases the amount of ions produced from a given sample andincreases the effective size of the sample, thus increasing the accuracyand speed of analysis.

In the preferred form of my apparatus the slot in the inlet electrode isin line with the slit in the adjacent propelling electrode andpreferably lies in a plane defined by the slits in the collimatingelectrodes, i. e. the plurality of propelling electrodes employed toform the ion beam and project it into the analyzer tube. It has beenfound that this type of structure aids markedly in obtaining improvedresults.

Further improvements. in results can be obtained if the current of theelectron beam passing through the slot is maintained substantiallyconstant, for example, as indicated at an electron catcher at which theelectrons are discharged after passing through the slot. In thestructure of the invention practically all of the ion beam formed in theionization chamber passes through the slits into the analyzer, so thatthe amount of ions entering the analyzer is proportional to the totalelectron current as collected by the catcher.

It has been found that some types of ions formed in a mass spectrometertend to lodge upon dielectric surfaces within the apparatus and to buildup static charges on these surfaces. The

gradual accumulation of such charges from this and other sources affectsthe calibration of the apparatus by changing the focussing of ions inthe beam. The effect is particularly marked at the insulator which isemployed as a spacer between collimating or propelling electrodes, and Ihave discovered that focussing electrodes disposed between thecollimating electrodes may serve a double function if they areconstructed in the form of a shield which obstructs a major portion ofthe view between the ion beam and the insulator. In this manner theeffect of the static charge of the insulator upon the ion beam isminimized. Accordingly my invention also contemplates the shielding ofthe ion beam from the effect of static charges built up on insulatingsurfaces by the interposition of a conductive shield therebetween whichis electrically connected to a part of the apparatus maintained underpotential and particularly the combination which comprises collimatingelectrodes, each having apertures through which an ion beam ispropelled, an insulating spacer holding the electrodes apart, aplurality of focussing electrodes disposed respectively on oppositesides of the electron beam between the collimating electrodes andbetween the insulator and the beam, these focussing electrodes beingspaced from each other and from the collimating electrodes and theinsulator but of such size that they obstruct a major portion of theview between the insulator vand the beam.

The foregoing type of structure is of particular benefit when employedin conjunction with the slotted inlet electrode described previously.Preferably. the focusing electrodes are plates bent at the two ends intochannel shape and held in place (out of contact with each other andbetween the collimating electrodes) by leads projecting through theinsulator. Thus the focussing electrodes substantially enclose the ionbeam or 'ribbon passing through the slits.

'I'he invention will be understood more thoroughly inthelight of thefollowing detailed description, taken in conjunction with theaccompanying drawings in which:

Figs. l and 2'are longitudinal sections taken at right angles to eachother through the head of a mass Aspectrometer adapted to the practiceof my invention; 'and Fig. 3 is a schematic diagram of a massspectrometer equipped with the headof Figs. 1 and 2.

Referring 'to the drawings, 1t will be seen that connects with a gasinlet-tube II of insulating material. Within the head there is an inletelectrode I2 in the form of a metallic tube which communicates with Vthegas inlet conduit at its rear and which is partially closed at its frontend by a cylindrical electrode plate I3 having Va transverse vslot toaccommodate an electron beam I4.

The electron beam is produced by an electron gun I6 mounted outside ablock I1 which encloses an ionization chamber IB within which theslotted electrode plate is disposed. (See Figs. "1 and 2.)

VThe electron beam enters 'the ionization cham- The front oftheionization chamber is Vformed by a propelling electrode 22 (composed ofsegments "22X, `22Y) provided with a slit S1 that is 4 substantially inline with the slot in the inlet electrode but Vslightly offsettherefrom, since (due to the magnetic fe'ld) the ion paths 'arecycloidal. A second propelling electrode 23 provided with a slit S2 isdisposed in front of the first propelling electrode with the slit S2 inline with the slot in the inlet electrode and the slit Si. 'I'he twopropelling electrodes are spaced from each other by a ring 24 of quartzor similar insulating material within which is a focussing chamber 25containing an opposed pair of semicircular focussing electrodes 26, 2lwhich extend close to, but are separated` from, the two propellingelectrodes by adequate dielectric gaps. The

focussing electrodes are thus constructed to permit them to serve thedualfunction of focussing the ion beam passing 'through 'the slits S1,S2 and of shielding vthe beam from the eifects 'of vagrant ions whichtend to collect upon the insulating spacer ring.

Electrode segments 22X, 22Y are spaced from each a point spaced from itsinner periphery, the segments are conductively joined to the block I1which forms the wall of the ionization chamber I 8.

The ion beam is shielded from the effect of static charges built upon'the a shield ring 22B disposed inside the insulator ring,electrically connected separated from the electrode by a substantialgap.

The focussing chamber communicates 'with `an analyzing chamber 28 of themass spectrometer segments 22X, 22Y

Referring now to Fig. 3, it will be observed that'the head I0 anenvelope 30, through the wall of which lthe gas inlet tube Il projects.A high degree of vacuum is maintained vwithin the envelope by rmeans ofvacuum pumps '(not shown) connected in series with Va parallel .pair ofresistors or potentiometers 15,45A and a second pair I6, 46A which arein turn connected to .ground .at .the negative end of 'the network.

The inlet electrode .is connected to the positive side of the network.at thes'lidero'f fthe .potentiometer 44. 'The propellingl electrode isconnected tothe negative end of the network and to ground.- The leadsthrough which the foregoing connections are made pass through seals inthe wall of the envelope. Y

In the operation of the apparatus, a high degree of vacuumis establishedwithin the envelopeand a sample .of gas to be analyzed is admittedthrough the gas inlet tubel and passes; through the slot in the inletelectrode, where itis bombarded by the electronbeam. lFor optimumresults the current producing` this. electron beam should be maintainedconstant, which result can be obtained by observing the current at theelectron catcher by a conventional meter (not shown) and adjusting theelectron current (for example, by the means shown in co-pending`4application Serial No. 591,010, filed April 30, 1945, by Harold W.Washburn et al., now Patent No.'2,486.452.)

Molecules of the gas sample are Athus ionized and the resulting ions arepropelled as an unsorted ion beam from the inlet electrode through theslots Si, Sz into theI analyzer tube. There the unsorted beam isseparated into a plurality, of diverging homogeneous io-n beams B1, Bz,B3, for example by means of an electro-magnetic field produced in theanalyzertube 'by an electromagnet (not shown). Any one of the'divergin'gbeams mayj be focused on the exit slit at the end of the analyzer tube.so that it passes therethrough and strikes an ion collector. .Thecurrent thus established in the ion collector is larnplied and' recordedby-suitable apparatus and serves as an index of the quantity of thatparticular type of ions produced from the sample.

The position of the ion beams with respect to the exit slit (i. e. thefocussing) may be changed by varying the electro-magnetic iield in theanalyzer tube o`r by varying the propelling potentials. The lattermethod is preferred and may be practiced as follows:

The switch 42 in the energizing network is closed and the condenser I3is charged to 'an appropriate level. Then the switch -is opened 'and thecondenser is permitted to, discharge through the-potential dividingnetwork and the several electrodes in the head of the mass spectrometer.In this way the propelling potentials between the several electrodesgradually decay, while maintaining relative values.

It will Vbe observed, however, that the potentials may be adjusted bysuitably setting the potentiometers. Thus the potential at the inletelectrode may be adjusted by varying the setting of the potentiometer44. Likewise, the potentials at the two propelling electrode segments22X, ZZY can be varied by varying the setting of the poten- Y 45A, andthe potentials of the focussing electrodes 26, 21 may be. varied bymoving the sliders of the potentiometers 46A, 46. In this way the ionbeams may be adjusted to give optimum results at the exit slit and ioncollector. Thus by varying the potentials on the electrodes which definethe slit Si, the position of the ion beam may be shifted laterally withrespect to the slit and may be centered in the slit. By varying thepotentials on the electrodes 26, 21 the ions may be brought to a focusin the slit Sz and also the focus may be moved laterally with respect tothis slit.

I claim:

1. In a mass spectrometer, the combination which comprises an ionizationchamber, an inlet electrode disposed within the chamber and having aninner end provided with a passage for admission of molecules into thechamber, the end having a slot'transverse to the passage, a secondelectrode spaced from the inlet electrode, means for producing lanelectrical propelling potential between thefe'lectrodes in theionization chamber, and meansfor propelling an electron beam through theslot transverse to the passage. 2. In a mass spectrometer, thecombination which comprises an ionization chamber, an inlet electrodedisposed within the chamber and having an inner end provided with apassage for admission of molecules into'the chamber, the end having aslot transverse to the passage, a second electrode spaced from the inletelectrode, and Ahaving a slit therein'in line with the slot in the inletelectrode, means for propelling an electron beam through the slottransverse to the passage for forming ions therein, and means forproducing an electrical propelling potential between the electrodes inthe ionization chamber to force ions formed in the slot through the slitin the second electrode.

3. In a mass spectrometer, the combination which comprises an inletelectrode having an end provided with a passage for molecules, the endhaving a slot transverse to 'the passage, means for propelling anelectron beam through the slot transverse to the passage to ionizemolecules issuing therefrom, collimating electrodes each having anaperture therein through which an ion beam is propelled from the inletelectrode, an insulating spacer holding the collimating electrodesapart, a plurality of focussing electrodes disposed respectively onopposite sides of the ion beam between the collimating electrodes andbetween the insulator and the beam, these focussing electrodes beingspaced from each other and from the collimating electrodes' and theinsulator but of such size that they obstruct a major portion of theview between the insulator and the beam.

4. Apparatus according to claim 3 in which the apertures in thecollimating electrodes are slits in line with the slot in the inletelectrode.

5. In mass spectrometry involving the propulsion of ions from a firstelectrode to another by an electrical potential'establishedtherebetween, the improvement which comprises forming the ions bybombarding a relatively concentrated stream of molecules with electronsin a space within the first electrode and permitting these ions to passinto the space by diiusion.

6. In mass spectrometry involving the propulsion of ions from arstelectrode to a second electrode by an electrical` field, theimprovement which comprises forming the ions by electron bombardmentwithin the rst escape therefrom of molecules in restricted spaceelectrdrief'gand permitting them to into the field by diffusion.

'1. In mass spectrometry involving the collirnation of ions byestablishing an electrical field between electrodes provided withorifices through which the ions pass. the improvement which coinprisesforming the lons by electron bombardment immediately prior to theirintroduction to the field, causing the ions to enter the field bydiffusion, and imposing on the ions a potential having a vectortransverse to a line drawn from one orifice to another.

8. In a mass spectrometer having an ionization chamber and a firstpropelling electrode spaced from the ionization chamber by a spacingring of insulating material and electrically connected to the ionizationchamber through the ring at points spaced from the inner wall thereof,the

between the electrodes tion chamber provided with a .ing an troducingmolecules. into the ionization chamber,

improvement which comprises a, .conductiveshie'ld ring disposed aroundthe inner wall of the spaeing ring, overlapping the inner wall of .theionization chamber and spaced from the electrode; s

9. In a mass spectrometer having an ionization chamber provided with ajrst electrode having .anelongated ion outlet slit, means forintroductrades.

10. In a mass spectrometer having yan .ionizajrst electrode havelongated4ion outlet slit, means for 'zinmeans for lionizing the molecules in thechamber and meansfor propellingions from the ionization chamber throughthe slit, the combination which comprises a second electrode spacedfromthe first electrode and provided between said first and second 11.Apparatus according to claim 1,0 wherein the two auxiliary electrodesvits Uses by Walter and afminor component transverse to the -major'azisiofthe slits.

tion chamber providedwith a ,rstelectrode having an elongated ion outletslit, means for introfrst electrode having an elongated ion .outletslit, means for introducing molecules into the :ionization chamber,meansjor ionizing-fthe molecules in the chamber and'meansjor propellingions from the ionization chamber through the slit, the combination whichcomprisesa second electrodespaced from the first REFERENCES CITED Thefollowing references are of record in the file of :this patent or-theoriginal patent:

`UNITED STATES `PATENTS Date Number .Name 2,354,122 .Hipple July `18,1944 12,472,870 Washburn AJune 14, 1949 OTHER REFERENCES Tech.Publication, The Mass Spectrograph ksind Bleakney in American PhysicsTeacher, volume 4, pages 12-23, February, 1936. l

